Radio communication systems (of voice or data signals) for trains having a lead unit and one or more remote control units or groups of remote control units are known. This arrangement where the locomotives are distributed within the train consist is referred to as distributed power operation and thus the communication system is referred to as a distributed power communication system. Generally, the one or more remote units or groups of remote units are controlled by commands from the lead unit carried over the communication system. The radio communication channel also carries responses by the remote units to the lead unit commands. In addition, certain important alarm conditions in the remote units and operational parametric data are brought to the attention of the engineer in the lead unit to ensure accurate and safe train operation.
Since many of the messages communicated between the lead and remote units in the moving train relate to proper traction and braking commands, they must be reliably and accurately received, even under a variety of changing operational and environmental conditions that affect the reliability of the communication link. Also, accuracy and reliability are required for signals communicated between the train and various land-based sites, such as a dispatching center, a locomotive monitoring and diagnostic center, personnel in a rail yard or in a loading/unloading facility and wayside equipment.
FIG. 1 schematically illustrates a train 18 and a distributed power communication system 10 for carrying control and monitoring signals between one or more remote units 12 and 13 and a lead unit 14 (FIG. 1). In another embodiment, not shown, the function performed by the lead unit 14 is replaced by a control tower where commands are issued (directly of via the lead unit) by a dispatcher to all locomotives in the train consist. An off-board repeater 26 may be disposed within radio communication distance of the train 18 for relaying communication signals transmitted between the lead unit 14 and the remote units 12 and 13 when direct communication between the lead unit 14 and the remote units is hampered, such as while the train 18 is traveling through a tunnel. The lead unit 14, the remote units 12 and 13, the off-board repeater 26 and the control tower (not shown) are provided with a transceiver 28 (and an antenna 29) for receiving and transmitting the communication signals. The lead unit transceiver 28 is associated with a lead controller 30 for issuing commands to control the remote units 12 and 13 and for responding to incoming signals from the remote units 12 and 13. Each of the remote units 12 and 13 and the off-board repeater 26 includes a remote controller 32 responsive to a signal from the transceiver 28 of the lead unit 14 for responding to the lead unit commands. The controller 32 can also initiate the transmission of messages to the lead unit 14 to advise of status information and alarm conditions.
In one embodiment of the existing railroad communication system, the communication link is a single half-duplex communication channel with a three KHz bandwidth. Each message transmission comprises a serial bit stream code word, further comprising information bits and error detecting bits derived from a geometrical error detecting scheme, modulating a carrier signal according to known frequency-shift keying modulation techniques. The types, contents and format of the various messages carried over the communication system 10 are described in detail in the commonly owned U.S. Pat. Nos. 5,039,038 and 4,582,580, both entitled Railroad Communication System, which are incorporated by reference herein. The system elements and message formats were intended to provide a secure transmission link for the information signals, with a low probability of accepting a message corrupted during transmission. The system also was intended to offer a low probability of interference from other lead and remote units on the same radio channel and within radio transmission distance.
The train 18 further includes a plurality of cars 20 that separate the lead unit 14 from the remote units 12 and 13. The cars 20 are coupled by a brake pipe 22 that signals a brake application in response to a drop in brake pipe pressure and a brake release in response to a pressure increase. The pressure in the brake pipe is controlled by an air brake controller 24 in the lead unit 14 and any or all of the remote units 12 and 13.
Each message includes information bytes or words and error detecting bits. As is known in the art, the inclusion of error detecting bits allows the receiver to detect bit errors that can occur during transmission over a noisy channel, at the expense of increasing the message overhead. In a prior art embodiment of the communication system 10, the error detecting bits are constructed in a geometrical format as horizontal and vertical parity bits. Each information byte is checked for odd or even parity, and as required an extra bit is added to satisfy the odd or even parity condition. This extra bit is referred to as a horizontal parity bit. Each message also includes a vertical parity byte that is generated to create a selectable odd or even parity for each column in the message, where the columns are formed by juxtaposing the words or bytes in overlying rows and determining the bit parity in each column. Once the parity of each column is determined, the vertical parity byte is formed to provide odd or even parity for the column.
An example is shown in FIG. 2 where the individual bits for information words A through D are set forth. These bits are merely illustrative and are not intended to represent a complete message carried over the communication system 10. Each word has eight information bits, labeled 0 through 7. The column labeled “HP” is the horizontal parity bit. In the example, odd parity is selected and therefore the value in the HP column is selected to ensure that an odd number of ones appear in each row, or that each of the bytes has odd parity. The vertical parity is established by the word in the “VP” row and in this example is selected to ensure that an even number of ones appear in each column.
The horizontal and vertical parity bits are employed at the receiving unit of the communication system 10 to determine whether errors occurred in the message as it traversed the channel. Upon receipt of a message by the lead unit 14 or a remote unit 12 or 13, the associated transceiver 28 demodulates the received signal into a baseband binary serial data stream and segregates the data stream into individual bytes. The applicable controller 30 or 32 determines the horizontal parity of the demodulated bytes as they are segregated. To determine the vertical parity, the bytes are arranged into a block form such as illustrated in FIG. 2. (Note that the formation of the code block of FIG. 2 is merely illustrative. It is not necessary to construct the block as the vertical parity can be determined by buffering individual bits such that buffered bits can be analyzed as though they were oriented in a column.) If the received words have the correct vertical and horizontal parity, the command or message represented by the baseband information segment is executed. If the parity is incorrect, the receiving unit rejects the message and a response is not transmitted. If the initiating lead unit (or the tower) does not receive a valid response from each unit to which the message was targeted, the message is retransmitted. At the receiving unit the retransmitted message is again demodulated and processed through the error detecting steps.
During most communication intervals the train is in motion. Thus the communication link can be lost or degraded when man-made or natural structures interfere with the communication path between the transmitting and receiving units as the train traverses the track. Also, the communication signal can be disrupted when the line-of-sight is lost between the transmitting and receiving units. Such link disruptions can cause errors in the received message. It has been observed during operation of the prior art distributed power communication system as described above, that certain four-bit errors in the received message may not be detected. There is also a statistically significant probability of not detecting errors with more than four erred bits. If errors go undetected at the receiving unit, train operational problems may arise. For example, if the lead unit 14 transmits a brake application command to the remote units, and the command is corrupted during transmission, but the corruption created errors in an undetectable error pattern, then the command is interpreted as a valid command, but a brake application is not made at the remote units 12 and 13.
An example of undetected errors that can occur with the prior art geometrical parity scheme is illustrated in FIG. 3. One class of undetectable errors cause an even number of errors in an even number of rows, where the errors occur in the same columns within each row. For simplicity, FIG. 3 illustrates only five rows of information words, each word comprising eight bits, plus a horizontal parity bit (HP) and a vertical parity bit (VP). Odd parity was selected. The erred bits are stricken and the value of the erred bit written above the strike mark. An error occurs when a transmitted zero bit is received as a one bit (or vice versa) due to noise and other channel effects. As can be seen by checking the parity of each row and column after occurrence of the indicated errors, the horizontal and vertical parity bits still indicate five correct information words, notwithstanding four erred bits. Such undetected errors can cause serious operational problems as the remote units 12 and 13 will not receive the correct command or data as transmitted from the lead unit 14, but are unable to determine that an error occurred.
Implementation of additional error detection capability to reduce the bit error probability is constrained by the large number of operational locomotives (lead units 14 and remote units 12 and 13) currently utilizing the prior art geometric parity scheme described above and the requirement that all locomotives in a fleet must be interoperable. The process of assembling a train having multiple locomotives so as to have sufficient motive power to meet the train's mission requirements is a complex one that would be made more difficult by the additional issue of locomotive communication system interoperability. Thus it is not possible to reduce the bit error rate by simply upgrading the communication system 10 of just isolated, individual locomotives to include one of the known more powerful error detecting methods, as the locomotives operating with the legacy geometric parity scheme would then not interoperate with the locomotives employing the newer error detecting scheme. Upgrading all locomotives throughout the entire North American railway network over a short period of time is problematic. Such a conversion would be a time-consuming, burdensome and expensive task in light of the large number of locomotives involved, their geographic dispersion and the various railroad company owners.
The problem thus remains to provide an error detecting scheme that reduces the bit error rate below that provided with the existing geometric parity scheme, while providing interoperability with communication systems employing the existing geometric parity scheme.